Power supply current monitoring device

ABSTRACT

A power supply current monitoring device is used for a load drive apparatus with two systems to drive a load. Each system includes a drive circuit connected in parallel with a battery, a capacitor connected between the battery and the drive circuit, and a relay connected between the drive circuit and a point at which power of the battery is divided between the systems. When overcurrent is detected once in one system, a repetitive monitoring process is performed. The monitoring process finally determines that the overcurrent actually occurs when a predetermined condition is satisfied after repeating a monitoring cycle in which an overcurrent time during which the overcurrent continues after the relay is turned ON is accumulated. The condition is satisfied when the monitoring cycle in which the overcurrent time reaches a predetermined threshold is repeated a predetermined consecutive number of times.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and claims priority to Japanese PatentApplication No. 2013-219110 filed on Oct. 22, 2013, the contents ofwhich are incorporated herein by reference.

FIELD

The present disclosure relates to a power supply current monitoringdevice for monitoring a magnitude of a power supply current in a drivecircuit which drives an electric load.

BACKGROUND

In a technique disclosed in JP-A-2003-219675, a power supply currentsensor is provided in a power supply path from a battery (i.e., a powersupply) to a drive circuit which drives an electric load such as amotor. When a short-circuit failure occurs in the drive circuit, thepower supply current sensor detects an overcurrent condition, and apower supply relay is turned OFF so that the power supply path can beinterrupted.

When a power supply current sensor is used to detect an overcurrentcondition, a detection error may be caused, for example, by noise or ashort circuit due to a momentary circuit contact. To eliminate such adetection error, a repetitive monitoring process may be performed. Inthe repetitive monitoring process, a power supply relay is repeatedlyturned ON and OFF after an overcurrent condition is detected once, andthen when the overcurrent condition is detected a predeterminedconsecutive number of times, it is finally determined that ashort-circuit failure occurs. For a drive circuit with one system havingan inverter, this repetitive monitoring process is effective ateliminating the detection error, thereby preventing an unnecessaryfail-safe operation from stopping functions.

For example, in an electric power steering system for a vehicle,multiple systems, each of which has a motor drive circuit, areconfigured in a redundant manner to increase reliability. In general, aninput stage of a drive circuit such as an inverter is provided with acapacitor to smooth an input voltage. In an apparatus where a drivecircuit of each of multiple systems is connected in parallel to a powersupply, a capacitor of an input stage of each drive circuit remainscharged during normal operation.

Here it is assumed that a power supply relay is turned OFF when anovercurrent condition in a drive circuit of one of two systems isdetected once. The system where the overcurrent condition is detectedonce is sometimes hereinafter referred to as the “temporary abnormalsystem”, while the other system is sometimes hereinafter referred to asthe “normal system”. If the drive circuit of the temporary abnormalsystem is actually short-circuited, the capacitor of the input stage ofthe temporary abnormal system is discharged. For this reason, when thepower supply relay is turned ON again in the repetitive monitoringprocess, charges stored in the capacitor of the input stage of thenormal system flows as an inrush current into the temporary abnormalsystem. Therefore, even when the short-circuit failure is cured so thatthe temporary abnormal system returns to normal, the repetitivemonitoring process incorrectly determines that the temporary abnormalsystem remains in an overcurrent condition due to the inrush current.That is, the inrush current is incorrectly detected as an excessivepower supply current.

Further, in the normal system, when the power supply relay is turned OFFupon the incorrect determination that the temporary abnormal systemremains in an overcurrent condition, a secondary inrush current flowsfrom the power supply to the capacitor of the input stage. Due to thissecondary inrush current, it may be incorrectly determined that thenormal system is in an overcurrent condition. Therefore, in an apparatuswith multiple systems, each of which has a drive circuit, it isdifficult to finally determine that the temporary abnormal system is inan overcurrent condition by the repetitive monitoring process.

SUMMARY

In view of the above, it is an object of the present disclosure toprovide a power supply current monitoring device capable of finallydetermining that a temporary abnormal system is in an overcurrentcondition by performing a repetitive monitoring process while preventinga detection error caused by an inrush current flowing from a normalsystem into the temporary abnormal system.

According to an aspect of the present disclosure, a power supply currentmonitoring device is used for a load drive apparatus which includes abattery and two systems configured to work in cooperation with eachother to drive a load. Each system includes a drive circuit, an inputcapacitor, and a power supply relay. The drive circuit is connected inparallel with the battery. The input capacitor is connected between thebattery and the drive circuit. The power supply relay is connectedbetween the drive circuit and a power blanching point, at which power ofthe battery is divided between the systems, to open and close a powersupply path from the battery to the drive circuit. The power supplycurrent monitoring device monitors a magnitude of a power supply currentflowing through the power supply relay toward the drive circuit in eachsystem. It is noted that the power supply current includes not only acurrent supplied from the battery to the drive circuit but also acurrent flowing from one system into the other system via the powerblanching point.

The power supply current monitoring device includes a failuredeterminator and a power supply relay controller. The failuredeterminator determines whether a short-circuit failure occurs based ona voltage across a current detector through which the power supplycurrents flows. The short-circuit failure is defined as a failureoccurring when a high potential side and a low potential side of thedrive circuit is short-circuited. The power supply relay controlleropens and closes the power supply relay.

When the failure determinator detects the power supply current exceedinga predetermined first threshold once in one of the systems, a repetitivemonitoring process is performed. The one of the systems is defined as atemporary abnormal system having a possibility that the short-circuitfailure occurs therein, and the other of the systems is defined as anormal system having no possibility that the short-circuit failureoccurs therein. The repetitive monitoring process determines that theshort-circuit failure actually occurs in the temporary abnormal systemwhen a predetermined condition is satisfied by repeating a monitoringcycle a predetermined number of times. In the monitoring cycle, thepower supply relay of the temporary abnormal system is turned OFF, andthen the magnitude of the power supply current is monitored when apredetermined cycle time elapses from a start time by turning ON thepower supply relay of the temporary abnormal system. The start time is atime at which the power supply current exceeds the first threshold.

The power supply current to be monitored in the monitoring cycle whenthe power supply relay of the temporary abnormal system is turned ONcontains a capacitor inrush current and a battery short-circuit current.The capacitor inrush current is a current flowing from the inputcapacitor of the normal system into the temporary abnormal system. Thebattery short-circuit current is a current flowing from the battery intothe temporary abnormal system when the short-circuit failure continuesin the temporary abnormal system.

The monitoring cycle accumulates an overcurrent time during which thepower supply current remains above the first threshold after the powersupply relay is turned ON. The condition is satisfied when themonitoring cycle in which the overcurrent time reaches a predeterminedsecond threshold is repeated a predetermined consecutive number oftimes.

The second threshold is greater than a first overcurrent time andsmaller than a sum of the first overcurrent time and a secondovercurrent time. The capacitor inrush current remains above the firstthreshold during the first overcurrent time, and the batteryshort-circuit current remains above the first threshold during thesecond overcurrent time.

Thus, in the repetitive monitoring process, whether or not theshort-circuit failures occurs in the temporary abnormal system can bedetermined based on presence or absence of the battery short-circuit byeliminating the influence of the capacitor inrush current on thedetermination. Since the battery short-circuit current does not flowwhen the temporary abnormal system returns to normal after theshort-circuit failure occurs temporarily, it is not finally determinedthat the short-circuit failure occurs in the temporary abnormal system.Further, even when noise is detected incorrectly as the power supplycurrent exceeding the first threshold once, it is not finally determinedthat the short-circuit failure occurs in the temporary abnormal systembecause neither the capacitor inrush current nor the batteryshort-circuit current flows in this case. Therefore, the power supplycurrent monitoring device can finally determine that the short-circuitfailure occurs in the temporary abnormal system by performing therepetitive monitoring process while preventing a detection error causedby the capacitor inrush current flowing from the normal system into thetemporary abnormal system.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a block diagram of a motor drive apparatus including a powersupply current monitoring device according to a first embodiment of thepresent disclosure;

FIG. 2 is a schematic diagram of the power supply current monitoringdevice;

FIG. 3 is a diagram for explaining a current flowing when ashort-circuit failure occurs in one system;

FIGS. 4A-4C show waveforms of a power supply current flowing through apower supply relay of a temporary abnormal system;

FIG. 5A is a schematic diagram of a circuit model used to obtain atheoretical waveform of a capacitor inrush current, and FIG. 5B is adiagram of the theoretical waveform of the capacitor inrush current;

FIG. 6A is a schematic diagram of a circuit model used to obtain atheoretical waveform of a battery short-circuit current, and FIG. 6B isa diagram of the theoretical waveform of the battery short-circuitcircuit;

FIG. 7 is a timing diagram of a repetitive monitoring process accordingto the first embodiment observed when a short-circuit failure occurredin one system continues;

FIG. 8 is a timing diagram of the repetitive monitoring process observedwhen a short-circuit failure occurred in one system is cured;

FIG. 9 is a timing diagram of the repetitive monitoring process observedwhen noise occurs in the power supply current monitoring device;

FIG. 10 is a flowchart of the repetitive monitoring process; and

FIG. 11 is a timing diagram of a repetitive monitoring process accordingto a second embodiment of the present disclosure observed when ashort-circuit failure occurred in one system continues.

DETAILED DESCRIPTION

Embodiments of the present disclosure are described below with referenceto the drawings. In the embodiments, a power supply current monitoringdevice according to the present disclosure is used in a motor driveapparatus for an electric power steering system for a vehicle.

First Embodiment

A motor drive apparatus 10 according to a first embodiment of thepresent disclose is described below with reference to FIGS. 1-10.Firstly, an overall structure of the motor drive apparatus 10 isdescribed with reference to FIG. 1.

(System Structure)

As shown in FIG. 1, the motor drive apparatus 10 includes amicrocomputer 20, a power supply current monitoring device 30, a battery47, and a power converter 50. The power converter 50 has a first-systeminverter 601 and a second-system inverter 602 to provide a redundantconfiguration. Each of the first-system inverter 601 and thesecond-system inverter 602 corresponds to a drive circuit recited inclaims. The motor drive apparatus 10 drives a motor 80 as an electricload. The motor drive apparatus 10 corresponds to a load drive apparatusrecited in claims. The motor drive apparatus 10 is used, in particular,for an electric power steering system that helps a driver steer avehicle by augmenting steering effort of a steering wheel 91.

The motor drive apparatus 10 controls and drives the motor 80 based onsignals including a steering torque signal and a rotation angle signal.The steering torque signal is inputted to the microcomputer 20 from atorque sensor 94. The rotation angle signal is inputted to themicrocomputer 20 from a rotation angle sensor 85. Accordingly, the motor80 outputs steering assist torque that augments the driver's steeringeffort of the steering wheel 91. The steering assist torque istransmitted to a steering shaft 92 through a reduction gear (not shown).

For example, the motor 80 is a three-phase brushless AC motor andincludes two three-phase winding sets: a first-system winding set 801and a second-system winding set 802. The power converter 50 has thefirst-system inverter 601 for the first-system winding set 801 and thesecond-system inverter 602 for the second-system winding set 802. Themotor 80 is driven by electric power outputted by the inverters 601 and602. Even when one of the inverters 601 and 602 breaks down, the otherof the inverters 601 and 602 can continue to drive the motor 80. In thisway, each of the power converter 50 and the motor 80 has first andsecond systems configured in a redundant manner to improve reliabilityof the motor drive apparatus 10.

In the description below, basically, components for the first system aregiven three-digit numbers ending in “1” and components for the secondsystem are given three-digit numbers ending in “2” so that thefirst-system components and be distinguished from the second-systemcomponents.

The battery 47 is an electrical energy storage device capable of storingand supplying DC power. Examples of the battery 47 can include anelectric double-layer capacitor, a secondary cell such as a lithium-ionsecondary cell, etc. The DC power of the battery 47 is divided betweenthe first system and the second system at a power branching point 54 onthe positive side of the battery 47.

In the power converter 50, an input side of the inverter 601 is providedwith a power supply relay 551, a shunt resistor 571, and an inputcapacitor 581, and an input side of the inverter 602 is provided with apower supply relay 552, a shunt resistor 572, and an input capacitor582. The power supply relay 551, the shunt resistor 571, and the inputcapacitor 581 are identical in specification and electric performancewith the power supply relay 552, the shunt resistor 572, and the inputcapacitor 582, respectively.

The power supply relay 551 is connected between the power branchingpoint 54 and the inverter 601 to connect and disconnect the inverter 601to and from a power supply path. The power supply relay 552 is connectedbetween the power branching point 54 and the inverter 602 to connect anddisconnect the inverter 602 to and from the power supply path.

The shunt resistor 571 is connected in series with the power supplyrelay 551, and the power supply current monitoring device 30 measures apower supply current flowing through the power supply relay 551 based ona voltage across the shunt resistor 571. The shunt resistor 572 isconnected in series with the power supply relay 552, and the powersupply current monitoring device 30 measures the power supply currentflowing through the power supply relay 552 based on a voltage across theshunt resistor 572. The power supply current has a positive value whenflowing in a direction from the battery 47 toward the inverters 601 and602. As described later, the power supply current monitoring device 30monitors the magnitude of the power supply current having the positivevalue for each system. The shunt resistors 571 and 572 correspond tocurrent detectors recited in claims.

The input capacitor 581 is connected between a high potential side and alow potential side of the inverter 601 to store charges, to assist powersupply to the inverter 601, and to smooth a ripple. The input capacitor582 is connected between a high potential side and a low potential sideof the inverter 602 to store charges, to assist power supply to theinverter 602, and to smooth a ripple.

The inverters 601 and 602 convert DC power of the battery 47 tothree-phase AC power and supply the AC power to the winding sets 801 and802, respectively.

The microcomputer 20 calculates control values, related to controllingand driving of the motor 80, based on the input signals including thesteering torque signal and the rotation angle signal.

The power supply current monitoring device 30 performs a repetitivemonitoring process as described later and includes a failuredeterminator 311 and a power supply relay pre-driver 391 for the firstsystem and a failure determinator 312 and a power supply relaypre-driver 392 for the second system. Each of the failure determinators311 and 312 corresponds to a failure determinator recited in claims.Each of the power supply relay pre-drivers 391 and 392 corresponds to apower supply relay controller recited in claims.

The failure determinator 311 detects the power supply current based onthe voltage across the shunt resistor 571 and determines whether thefirst system suffers from a short-circuit failure that occurs when thehigh potential side and the low potential side of the inverter 601 areshort-circuited. The failure determinator 312 detects the power supplycurrent based on the voltage across the shunt resistor 572 anddetermines whether the second system suffers from a short-circuitfailure that occurs when the high potential side and the low potentialside of the inverter 602 are short-circuited. The high potential side ofeach of the inverters 601 and 602 is connected to a positive terminal ofthe battery 47, and the low potential side of each of the inverters 601and 602 is connected to a negative terminal of the battery 47. Asdescribed above, according to the first embodiment, the “short-circuitfailure” is defined as a failure occurring when the high potential sideand the low potential side of the drive circuit are short-circuited anddoes not mean a failure occurring, for example, when phases of thethree-phase winding set are short-circuited.

The power supply relay pre-driver 391 closes and opens the power supplyrelay 551 based on commands from the failure determinator 311 and themicrocomputer 20. The power supply relay pre-driver 392 closes and opensthe power supply relay 552 based on commands from the failuredeterminator 312 and the microcomputer 20. Further, the power supplyrelay pre-drivers 391 and 391 exchange signals related to a maskprocedure each other. The mask procedure is described later.

Next, a detailed structure of the power supply current monitoring device30 is described with reference to FIG. 2. Like the power converter 50,the power supply current monitoring device 30 has first and secondsystems configured in a redundant manner to improve reliability of themotor drive apparatus 10. Since the first system and the second systemof the power supply current monitoring device 30 are structured in thesame manner, a structure of the first system is described as an example.

As described above, the power supply current monitoring device 30includes the failure determinator 311 and the power supply relaypre-driver 391. The failure determinator 311 determines whether theshort-circuit failure occurs in the inverter 601. The power supply relaypre-driver 391 opens and closes the power supply relay 551.

The failure determinator 311 includes a voltage amplifier 321, acomparator 331, an NAND gate 341, an accumulated time counter 351, anovercurrent determinator 361, and a diagnosis signal generator 371.

The voltage across the shunt resistor 571 as a detection voltage isamplified by the voltage amplifier 321 and then inputted to thecomparator 331. The comparator 331 compares the amplified detectionvoltage with a predetermined reference voltage. When the detectionvoltage is less than the reference voltage, the comparator 331 outputs alow signal, and when the detection voltage is not less than thereference voltage, the comparator 331 outputs a high signal.

The NAND gate 341 performs a logical NAND operation between the outputsignal of the comparator 331 and a clock signal and outputs a signalindicative of the result of the NAND operation to the accumulated timecounter 351.

The overcurrent determinator 361 determines whether an accumulated timecounted by the accumulated time counter 351 reaches a predeterminedthreshold time. When determining that the accumulated time reaches thethreshold time, the overcurrent determinator 361 outputs an overcurrentdetection signal.

The diagnosis signal generator 371 generates a diagnosis signal based ona result of the determination by the overcurrent determinator 361 andoutputs the diagnosis signal to the microcomputer 20.

To close and open the power supply relay 551, an AND gate 381 of thefirst system performs a logical AND operation on three inputs: thecommand from the microcomputer 20, the overcurrent detection signal fromthe overcurrent determinator 361, and a signal indicating that thesecond system is outside a predetermined monitoring time Tw, which isdescribed later. Likewise, to close and open the power supply relay 552,an AND gate 382 of the second system performs a logical AND operation onthree inputs: the command from the microcomputer 20, the overcurrentdetection signal from an overcurrent determinator 362, and a signalindicating that the first system is outside the monitoring time Tw. Inthis way, the power supply current monitoring device 30 performs themask procedure for one of the first and second systems based on whetherthe other of the first and second systems is within the monitoring timeTw.

The power supply relay pre-driver 391 transmits a switching signal to agate of the power supply relay 551 based on an output signal of the ANDgate 381, and the power supply relay pre-driver 392 transmits aswitching signal to a gate of the power supply relay 552 based on anoutput signal of the AND gate 382.

Next, a detailed structure of the power converter 50 and a currentflowing in the power converter 50 when the short-circuit failure occursin any of the first system and the second system are described withreference to FIG. 3 and FIGS. 4A to 4C.

As shown in FIG. 3, the power converter 50 has a positive terminalconnection point 51 connected to a positive wire extending from thepositive terminal of the battery 47 and a negative terminal connectionpoint 59 connected to a negative wire extending from the negativeterminal of the battery 47. Each of the positive and negative wires hasa wire resistance 48 and a wire inductance 49.

The power converter 50 has a capacitor 52 and a coil 53 on an input sideleading to the positive terminal connection point 51. The capacitor 52and the coil 53 form a noise filter. The power branching point 54, atwhich the DC power of the battery 47 is divided between the first systemand the second system, is provided by a terminal of the coil 53 on a farside from the battery 47. The first system, shown on a left side of FIG.3, includes the power supply relay 551, the shunt resistor 571, theinput capacitor 581, and the inverter 601. The second system, shown on aright side of FIG. 3, includes the power supply relay 552, the shuntresistor 572, the input capacitor 582, and the inverter 602.

According to the first embodiment, each of the power supply relays 551and 552 has a switching element, and each of the inverters 601 and 602has six switching elements connected in a bridge configuration. Theswitching elements of the inverter 601 are turned ON and OFF inaccordance with the switching signals inputted to their gates from thepower supply relay pre-driver 391 so that each phase of the three-phasewinding set 801 can be energized in turn. The switching elements of theinverter 602 are turned ON and OFF in accordance with the switchingsignals inputted to their gates from the power supply relay pre-driver392 so that each phase of the three-phase winding set 802 can beenergized in turn. For example, these switching elements can bemetal-oxide semiconductor field-effect transistors (MOSFETs). Since astructure of a three-phase inverter is well known, detailed explanationof the structures of the inverters 601 and 602 are omitted.

Here it is assumed that the short-circuit failure occurs in the powerconverter 50 due to the fact that the high potential side and the lowpotential side of the inverter 601 of the first system areshort-circuited. FIG. 3 shows the short-circuit failure schematicallyusing an imaginary switch SC. It can be considered that a situationwhere the imaginary switch SC is ON is equivalent to a situation wherethe short-circuit failure occurs.

In the following description based on the situation shown in FIG. 3, thefirst system, where the short-circuit failure occurs, is sometimesreferred to as the “abnormal system”, and the second system is sometimesreferred to as the “normal system”.

When the short-circuit failure occurs in the first-system inverter 601,a battery short-circuit current Ib from the battery 47 flows through thefirst-system power supply relay 551 as indicated by a solid arrow.Further, charges stored in the second-system input capacitor 582 flowsas a capacitor inrush current Ic through the first-system power supplyrelay 551 as indicated by a broken arrow. Accordingly, the batteryshort-circuit current Ib and the capacitor inrush current Ic arecollectively detected as the “power supply current” of the first system.

As described above, in the power converter 50 where the inverters 601and 602 of the two systems are connected in parallel with the battery47, when the short-circuit failure occurs in one system, the capacitorinrush current Ic flows from the normal system into the abnormal systemby way of the power branching point 54. This phenomenon is unique to astructure with multiple-system inverters and cannot be expected in astructure with a single-system inverter.

In general, when a power supply current sensor is used to detect anexcessive power supply current, a detection error may be caused, forexample, by noise or a short circuit due to a momentary circuit contact.To eliminate such a detection error, a repetitive monitoring process maybe performed. In the repetitive monitoring process, a power supply relayis repeatedly turned ON and OFF after the excessive power supply currentis detected once, and when the excessive power supply current isdetected a predetermined consecutive number of times, it is determinedthat a short-circuit failure occurs. For a drive circuit with asingle-system inverter, this repetitive monitoring process is effectiveat eliminating the detection error, thereby preventing an unnecessaryfail-safe operation from stopping functions.

However, in an apparatus with two systems, each of which has a drivecircuit, it is difficult to adequately perform the repetitive monitoringprocess because of the above-described capacitor inrush current Ic. Toexplain a reason for this, changes in the capacitor inrush current Icand the battery short-circuit current Ib over time in the repetitivemonitoring process are described with reference to FIGS. 4A-4C. In therepetitive monitoring process, a system where an excessive power supplycurrent is detected once is regarded as the “temporary abnormal system”,and the power supply current is monitored by repeatedly turning ON andOFF a power supply relay. Thus, the repetitive monitoring processdetermines whether the excessive power supply current is detected due toa short-circuit failure actually occurring in an inverter of thetemporary abnormal system or due to other factors such as noise, amomentary contact, etc.

FIG. 4A shows a waveform of a power supply current obtained in anexperiment conducted by the present inventor. FIG. 4B shows a powersupply current I of the temporary abnormal system schematically. FIG. 4Cshows a power supply current J of the normal system schematically. FIGS.4A-4C are based on a situation where the short-circuit failure actuallyoccurs in the temporary abnormal system.

In the temporary abnormal system, when the power supply relay 551 isturned ON at the time t0, so that a short-circuit condition occurs, apeak of the capacitor inrush current appears firstly, and subsequently apeak of the battery short-circuit current appears. Then, when the powersupply current exceeds a predetermined threshold Vth, it is detectedthat the power supply current is excessive.

As described previously, in the power supply current monitoring device30, the voltage across each of the shunt resistors 571 and 572 as thedetection voltage is amplified and then compared with the referencevoltage. That is, in fact, the power supply current is not directlycompared with a current threshold. However, it is a common knowledgethat a voltage and a current can be converted to each other. Therefore,in the first embodiment, the expression “when the power supply currentexceeds the threshold Vth” is used.

As shown in FIG. 4B, in the temporary abnormal system, the capacitorinrush current Ic remains above the threshold Vth during a time periodfrom the time t1 to a time t2, and the battery short-circuit current Ibremains above the threshold Vth during a time period from a time t3 to atime t5. The time period during which the power supply current remainsabove the threshold Vth is hereinafter referred to as the “overcurrenttime”. Specifically, the time period from the time t1 to the time t2,during which the capacitor inrush current Ic remains above the thresholdVth, is hereinafter referred to as the “first overcurrent time Tc”, andthe time period from the time t3 to the time t5, during which thebattery short-circuit current Ib remains above the threshold Vth, ishereinafter referred to as the “second overcurrent time Tb”. Further,the time t1 at which the power supply current exceeds the threshold Vthfor the first time is hereinafter referred to as the “start time t1”.The accumulated time is counted from the start time t1.

In the temporary abnormal system, the power supply relay 551 is turnedOFF at a time t4 between the time t3 and the time t5 as a fail-safeoperation.

In contrast, as shown in FIG. 4C, in the normal system, a capacitordischarge current Jc having a negative value occurs at a time t0 as areverse of the capacitor inrush current Ic. Further, when the powersupply relay 551 of the temporary abnormal system is turned OFF at thetime t4 as described above, a terminal voltage of the power branchingpoint 54 increases by energy stored in the wire inductance 49 and thecoil 53. Accordingly, a current Jbc having a positive value flows fromthe battery 47 into the input capacitor 582. The current Jbc ishereinafter referred to as the “secondary inrush current”.

Next, a rationale for the capacitor inrush current Ic is described withreference to FIGS. 5A and 5B, and a rationale for the batteryshort-circuit current Ib is described with reference to FIGS. 6A and 6B.

As shown in FIG. 5A, in a circuit assumed here, a resistor having aresistance R and a coil having an inductance L are connected in serieswith a capacitor having a capacitance C and charged to a DC voltage E0.When the circuit changes from an open state to a closed change byclosing a switch as indicated by an arrow in FIG. 5A, a capacitor inrushcurrent Ic(t) given by the following formulas (1.1), (1.2), and (1.3) isdischarged from the capacitor:

$\begin{matrix}{{{Ic}(t)} = {{\frac{E_{0}}{L \cdot \omega_{f}} \cdot ^{- \frac{t}{\tau}}}\sin \; \omega_{f}t}} & (1.1) \\{\omega_{f} = \sqrt{\frac{1}{LC} - \left( \frac{R}{2L} \right)^{2}}} & (1.2) \\{\tau = \frac{2L}{R}} & (1.3)\end{matrix}$

As shown in FIG. 5B, a waveform of the capacitor inrush current Ic(t) issuch that when a time t is 0, the capacitor inrush current Ic is 0, whenthe time t is tp, the capacitor inrush current Ic has a positiveextremum, and when the time t is greater than tp, the capacitor inrushcurrent Ic converges to zero.

Next, as shown in FIG. 6A, in a circuit assumed here, a resistor havinga resistance R and a coil having an inductance L are connected in serieswith a battery of a DC voltage E0. Based on the circuit shown in FIG.6A, a battery short-circuit current Ib(t) is given by the followingformula (2):

$\begin{matrix}{{{Ib}(t)} = {\frac{E_{0}}{R}\left( {1 - ^{{- \frac{R}{L}}t}} \right)}} & (2)\end{matrix}$

As shown in FIG. 6B, the battery short-circuit current Ib (t) has awaveform of a step response of a first-order system.

In practice, a combined waveform of the battery short-circuit current Ib(t) and the capacitor inrush current Ic(t) appears in the inverters 601and 602.

Because of such a behavior of the power supply current as describedabove, the following phenomenon occurs in the repetitive monitoringprocess.

Firstly, it is assumed the temporary abnormal system returns to normalafter the power supply relay 551 is turned OFF once. Even when the powersupply relay 551 is turned ON again under this condition, the batteryshort-circuit current Ib does not flow. However, since the inputcapacitor 581 of the temporary abnormal system has been discharged whenthe power supply relay 551 was turned ON in the previous cycle, thecapacitor inrush current Ic flows from the input capacitor 582 of thenormal system when the power supply relay 551 is turned ON again. Due tothis capacitor inrush current Ic, the power supply current in thetemporary abnormal system is incorrectly detected as excessive. As aresult, although the temporary abnormal system returns to normal, it isdetermined that the short-circuit failure occurs in the temporaryabnormal system. Therefore, there is no point in performing therepetitive monitoring process.

Further, in the normal system, the capacitor inrush current Ic isdischarged from the input capacitor 582 when the power supply relay 551of the temporary abnormal system is turned OFF. Then, when the powersupply relay 551 of the temporary abnormal system is turned OFF upondetection that the power supply current is excessive, the secondaryinrush current Jbc flows from the battery 47 into the input capacitor582 of the normal system. Due to this secondary inrush current Jbc, thepower supply current in the normal system may be incorrectly detected asexcessive.

For the reasons described above, in an apparatus with multiple systems,each of which has a drive circuit, unlike an apparatus with one systemwhich has a drive circuit, it is difficult to adequately perform therepetitive monitoring process. According to the first embodiment, adetection error caused by the capacitor inrush current Ic, which flowsfrom the input capacitor 582 of the normal system into the temporaryabnormal system when the power supply relay 551 is turned OFF, isprevented as described below.

A repetitive monitoring process performed by the power supply currentmonitoring device 30 according to the first embodiment of the presentdisclosure is described below with reference to FIGS. 7, 8, 9, and 10.

FIG. 7 illustrates timing diagrams of a first case where theshort-circuit failure occurs in the temporary abnormal system andcontinues. FIG. 7 shows characteristic values in a first monitoringcycle, in a second monitoring cycle, and in a Nth monitoring cycle ofthe repetitive monitoring process when a predetermined number of timesthe repetitive monitoring process is to be performed is set to N, whereN is an integer not less than three (i.e., N≧3). Characteristic valuesin each of third to “N−1” monitoring cycles of the repetitive monitoringprocess is the same as those in the second monitoring cycle.

(a), (b), (c), (d), (g), and (h) of FIG. 7 show characteristic valuesrelated to the temporary abnormal system.

(a) of FIG. 7 corresponds to FIG. 4B and shows the power supply currentin the temporary abnormal system. As shown in (a) of FIG. 7, thewaveforms of the capacitor inrush current Ic and the batteryshort-circuit current Ib appear in each monitoring cycle.

(b) of FIG. 7 shows the output signal of the comparator 331. The outputsignal of the comparator 331 is high during a time period where thepower supply current shown in (a) of FIG. 7 remains above the thresholdVth.

(c) of FIG. 7 shows the accumulated time counted by the accumulated timecounter 351. The first overcurrent time Tc is accumulated while thecapacitor inrush current Ic flows after the start time t1. Anovercurrent determination threshold Tj is set to a value greater thanthe first overcurrent time Tc and smaller than the sum of the firstovercurrent time Tc and the second overcurrent time Tb. A margin periodTa is set to a value obtained by subtracting the first overcurrent timeTc from the overcurrent determination threshold Tj. That is, the firstovercurrent time Tc, the second overcurrent time

Tb, the overcurrent determination threshold Tj, and the margin period Tahave relationships given by the following formulas (3.1), (3.2), and(3.3).

Tc<Tj<Tc+Tb   (3.1)

Tc+Ta =Tj   (3.2)

Ta<Tb   (3.3)

The margin period Ta is a margin to the overcurrent determinationthreshold Tj at the time the first overcurrent time Tc is accumulated.In other words, the accumulated time counter 351 reaches the overcurrentdetermination threshold Tj when accumulating the margin period Ta afteraccumulating the first overcurrent time Tc. The accumulated time counter351 is reset to an initial value (e.g., zero) when a predeterminedmonitoring time Tw (e.g., 1 ms) elapses from the start time t1.

(d) of FIG. 7 shows an ON and OFF state of the power supply relay 551 ofthe temporary abnormal system. The power supply relay 551 is turned OFFwhen the accumulated time counter 351 reaches the overcurrentdetermination threshold Tj. Then, the power supply relay 551 is turnedON again when a predetermined cycle time Tx (e.g., 5 ms) elapses fromthe start time t1 under a condition that the number of times therepetitive monitoring process has been performed is less than thepredetermined number N. Thus, a next monitoring cycle starts at a timet0 at which the power supply relay 551 has been turned ON again.

Strictly speaking, the cycle time Tx is a time from a first time t0 atwhich the power supply relay 551 is turned ON firstly to a second timet0 at which the power supply relay 551 is turned ON again. However, atime from the first time t0 to the start time t1 at which the powersupply current exceeds the threshold Vth is very small and negligible.Therefore, for the sake of simplicity, the time from the start time t1to the second time t0 at which the power supply relay 551 is turned ONagain so that the next monitoring cycle can start is hereinafterreferred to as the “cycle time Tx”.

(g) and (h) of FIG. 7 show diagnosis signals outputted by the powersupply current monitoring device 30 to the microcomputer 20.Specifically, (g) of FIG. 7 shows a temporary diagnosis signal outputtedwhen the power supply relay 551 is turned OFF in each monitoring cycle,and (h) of FIG. 7 shows a final diagnosis signal outputted when thetemporary diagnosis signal is outputted a predetermined consecutivenumber N of times. When receiving the temporary diagnosis signal or thefinal diagnosis signal, the microcomputer 20 performs a predeterminedfailure handling operation for the temporary abnormal system from afail-safe standpoint.

(e) and (f) of FIG. 7 show characteristic values related to the normalsystem.

(e) of FIG. 7 shows a mask time during which a mask procedure to stopmonitoring the power supply current in the normal system is performed.According to the first embodiment, the mask time is from when the powersupply relay 551 is turned OFF to when the monitoring time Tw elapses ineach monitoring cycle. It is noted that the monitoring time Tw is setsmaller than the cycle time Tx.

(f) of FIG. 7 corresponds to FIG. 4C and shows the power supply currentin the normal system. The monitoring time Tw is set so that a periodwhere the secondary inrush current Jbc appears can be included in themask time.

The above matters for FIG. 7 hold for FIGS. 8 and 9.

In the first monitoring cycle, when the capacitor inrush current Ic andthe battery short-circuit current Ib flow in the temporary abnormalsystem, the accumulated time counter 351 reaches the overcurrentdetermination threshold Tj by accumulating the first overcurrent time Tcand the margin time Ta. Then, the power supply relay 551 is turned OFF,the temporary diagnosis signal is outputted, and the mask procedure isstarted.

Then, when the monitoring time Tw elapses after the start time t1, theaccumulated time counter 351 is reset, and the mask procedure isfinished. Then, when the cycle time Tx elapses from the start time t1,the power supply relay 551 is turned OFF again so that the secondmonitoring cycle can be started.

If the short-circuit failure continues, the same behavior as in thefirst monitoring cycle is repeated in each of the second to “N−1”thmonitoring cycles. Then, when the power supply relay 551 is turned OFFin the Nth monitoring cycle, i.e., when the power supply relay 551 isturned OFF for the Nth time in total, the final diagnosis signal istransmitted to the microcomputer 20 in addition to the Nth temporarydiagnosis signal. Thus, it is finally determined that the short-circuitfailure occurs in the temporary abnormal system.

FIG. 8 illustrates timing diagrams of a second case where theshort-circuit failure temporarily occurs in the temporary abnormalsystem and then the temporary abnormal system returns to normal. Thefirst monitoring cycle, which is performed for the first time after theshort-circuit failure occurs in the temporary abnormal system, is thesame between the first case shown in FIG. 7 and the second case shown inFIG. 8.

However, as shown in (a) of FIG. 8, when the temporary abnormal systemreturns to normal after the monitoring time Tw elapses for the firsttime, although the capacitor inrush current Ic flows in the temporaryabnormal system in the second monitoring cycle, the batteryshort-circuit current Ib does not flow.

Accordingly, the accumulated time counter 351 accumulates only the firstovercurrent time Tc. Therefore, as shown (c) of FIG. 8, before theaccumulated time counter 351 reaches the overcurrent determinationthreshold Tj, the monitoring time Tw elapses so that the accumulatedtime counter 351 is reset. Therefore, the power supply relay 551 whichis turned ON in the second monitoring cycle remains ON. Thus, therepetitive monitoring process for the temporary abnormal system isautomatically ended when the monitoring time Tw elapses. In the normalsystem, the secondary inrush current Jbc from the battery 47 does notoccur because the power supply relay 551 remains ON.

FIG. 9 illustrates timing diagrams of a third case where theshort-circuit failure actually occurs in neither system, but noise ordisturbance enters any of the systems. Here, it is assumed that thenoise is detected in the first system. For the sake of convenience,although the first system in which the noise is detected is hereinafterreferred to as the “temporary abnormal system”.

As shown in (a) and (f) of FIG. 9, a short-circuit current actuallyoccurs in neither system. However, as shown in (b) of FIG. 9, a pulsenoise of a few μs appears in the output signal of the comparator 331. Inthis case, as shown in (c) of FIG. 9, the accumulated time counter 351starts to accumulate the time in response to a first pulse of the noise.However, since the width of the pulse is very small, the timeaccumulated by the accumulated time counter 351 becomes very smallaccordingly. Therefore, before the accumulated time counter 351 reachesthe overcurrent determination threshold Tj, the monitoring time Twelapses so that the accumulated time counter 351 is reset.

In this way, it is possible to adequately prevent the noise from causingthe detection error.

FIG. 10 is a flowchart of the repetitive monitoring processcorresponding to the timing diagrams shown in FIGS. 7 and 8.

The repetitive monitoring process starts at S11 where it is determinedwhether the power supply current in the temporary abnormal system isexcessive. It is noted that at S11 of the first monitoring cycle of therepetitive monitoring process, it is determined whether the power supplycurrent in any system is excessive, and the system in which theexcessive power supply current flows is regarded as the “temporaryabnormal system”. Then, when the repetitive monitoring process returnsto S11 from S18 so that the next monitoring cycle can start, it isdetermined at S11 whether the power supply current in the temporaryabnormal system remains excessive.

If the power supply current in the temporary abnormal system isexcessive corresponding to YES at S11, the repetitive monitoring processproceeds to S12 where the overcurrent time starts to be accumulated fromthe start time t1 at which the power supply current exceeds thethreshold Vth. If the short-circuit failure continues after that, thecapacitor inrush current Ic and the battery short-circuit current Ibflows in the temporary abnormal system (see, FIG. 7). In contrast, ifthe temporary abnormal system returns to normal after that, thecapacitor inrush current Ic flows, but the battery short-circuit currentIb does not flow (see, FIG. 8).

Then, the repetitive monitoring process proceeds to S13 where it isdetermined whether the accumulated time reaches the overcurrentdetermination threshold Tj. If the first overcurrent time Tc caused bythe capacitor inrush current Ic and the margin time Ta out of the secondovercurrent time Tb caused by the battery short-circuit current Ib areaccumulated so that the accumulated time can reach the overcurrentdetermination threshold Tj, it is determined that the accumulated timereaches the overcurrent determination threshold Tj corresponding to YESat S13, and the repetitive monitoring process proceeds to S14. At S14,the power supply relay 551 is turned OFF, and a relay OFF number, whichis the number of times the power supply relay 551 is turned OFF, isincremented by one. Further, at S14, the power supply current monitoringdevice 30 transmits the temporary diagnosis signal to the microcomputer20 (see, FIG. 7).

In contrast, if the first overcurrent time Tc and the margin time Ta arenot accumulated so that the accumulated time cannot reach theovercurrent determination threshold Tj before the monitoring time Twelapses from the start time t1, it is not determined that theaccumulated time reaches the overcurrent determination threshold Tjcorresponding to NO at S13, and the repetitive monitoring processproceeds to S20. At S20, the accumulated time counter is reset, and therepetitive monitoring process is ended (see, FIG. 8).

At S15 subsequent to S14, it is determined whether the relay OFF numberreaches the predetermined number N, which is the number of times therepetitive monitoring process is to be performed. If the relay OFFnumber does not reach the number N corresponding to NO at S15, therepetitive monitoring process proceeds to S16 where the mask procedurefor the normal system is started at the same time when the power supplyrelay is turned OFF. Then, the repetitive monitoring process proceeds toS17 where the accumulated time counter is reset and the making procedureis finished when the monitoring time Tw elapses from the start time t1.Then, the repetitive monitoring process proceeds to S18 where the powersupply relay is turned ON when the cycle time Tx elapses from the starttime t1. After S11, the repetitive monitoring process returns to S11 sothat the next monitoring process can be started (see, FIG. 7).

In contrast, if the relay OFF number reaches the number N correspondingto YES at S15, the repetitive monitoring process proceeds to S19 whereit is finally determined that the short-circuit failure occurs in thetemporary abnormal system, and the power supply current monitoringdevice 30 transmits the final diagnosis signal to the microcomputer 20.After S19, the repetitive monitoring process is ended.

In summary, the power supply current monitoring device 30 according tothe first embodiment of the present disclosure can have the followingadvantages (I)-(IV).

(I) In the repetitive monitoring process for the temporary abnormalsystem, the power supply current monitoring device 30 does not finallydetermine that the short-circuit failure occurs in the temporaryabnormal system until the monitoring cycle, in which the accumulatedtime of the overcurrent time reaches the determination threshold Tj, isrepeated the predetermined consecutive number N of times after the powersupply relay 551 is turned ON. The overcurrent determination thresholdTj is greater than the first overcurrent time Tc caused by the capacitorinrush current Ic and smaller than the sum of the first overcurrent timeTc and the second overcurrent time Tb caused by the batteryshort-circuit current Ib. In such an approach, whether or not theshort-circuit failure occurs can be determined based on whether or notthe battery short-circuit current Ib flows without consideration of thecapacitor inrush current Ic.

(II) Further, in the repetitive monitoring process for the temporaryabnormal system, the power supply current monitoring device 30 performsthe mask procedure to stop monitoring the power supply current in thenormal system. The mask procedure is performed at least for apredetermined time period after the power supply relay of the temporaryabnormal system is turned OFF in each monitoring cycle. In such anapproach, it is possible to prevent the power supply current in thenormal system from being incorrectly determined as excessive due to thesecondary inrush current Jbc.

(III) Further, in each monitoring cycle of the repetitive monitoringprocess, when the accumulated time of the overcurrent time reaches thedetermination threshold Tj after the power supply relay 551 is turnedON, the mask procedure is started at the same time when the power supplyrelay 551 is turned OFF. Then, the mask procedure is finished when themonitoring time Tw elapses from the start time t1. In this way, since atime period during which the mask procedure is performed is minimized, arisk of failing to detect the short-circuit failure occurring in thenormal system during the mask procedure can be minimized.

(IV) The power supply current monitoring device 30 according to thefirst embodiment is used in the motor drive apparatus 10 for an electricpower steering system for a vehicle. In general, to increase reliabilityof an electric power steering system, it is preferable that a motordrive apparatus for an electric power steering system should have twosystems, each of which a drive circuit, configured in a redundantmanner. Therefore, the power supply current monitoring device 30 caneffectively exert its effects, in particular, when used in the motordrive apparatus 10.

Second Embodiment

A repetitive monitoring process performed by a power supply currentmonitoring device 30 according to a second embodiment of the presentdisclosure is described below with reference to FIG. 11. FIG. 11corresponds to FIG. 7 and is illustrated with the same symbols as usedin FIG. 7.

As shown in (e) of FIG. 11, according to the second embodiment, the maskprocedure for the normal system is continuously performed from the starttime t1, at which the power supply current exceeds the threshold Vth forthe first time, to the end of the monitoring time Tw in the last (i.e.,Nth) monitoring cycle of the repetitive monitoring process. That is, themask procedure is continuously performed without being stopped in eachmonitoring cycle. Thus, a mask period, where the mask procedure isperformed, of the second embodiment includes the entire mask period ofthe first embodiment.

When a period from the time t0, at which the power supply relay isturned ON, to the start time t1 in each monitoring cycle is ignored, amask period of the second embodiment can be given as follows:Pm≈Tx×(N−1)+Tw. Procedures except the mask procedure are performed inthe same manner as in the first embodiment. For example, whether or notthe short-circuit failure occurs in the temporary abnormal system isdetermined based on the accumulated time.

As described above, according to the second embodiment, the maskprocedure is started at the start time t1 at which the power supplycurrent exceeds the threshold Vth for the first time. Alternatively,like in the first embodiment, the mask procedure can be started when thepower supply relay is turned OFF in the first monitoring cycle.

The advantages (I), (II), and (IV) of the first embodiment can beobtained in the second embodiment.

(Modification)

While the present disclosure has been described with reference to theembodiment, it is to be understood that the disclosure is not limited tothe embodiment. The present disclosure is intended to cover variousmodifications and equivalent arrangements within the spirit and scope ofthe present disclosure.

The current detector which produces a voltage based on which the powersupply current is detected is not limited to the shunt resistors 571 and572 but includes other elements having predetermined resistances. Asdescribed in JP-A-2013-183462, for example, to prevent currents fromflowing to the inverters 601 and 602 through parasitic diodes of thepower supply relays 551 and 552 when the battery 47 is connected inreverse polarity, additional power supply relays having parasitic diodesopposite in direction to those of the power supply relays 551 and 552can be connected in series. In this configuration, the additional powersupply relay can be used as the current detector.

In the embodiments, the power supply current monitoring device accordingto the present disclosure is applied to two inverters 601 and 602 whichare configured to work in cooperation with each other to drive athree-phase brushless AC motor. Alternatively, the power supply currentmonitoring device according to the present disclosure can be applied totwo H-bridge circuits which are configured to work in cooperation witheach other to drive a DC motor. These motors are not limited to asteering assist motor of an electric power steering system. Further, thepower supply current monitoring device according to the presentdisclosure can be applied to any types of drive circuits which areconfigured to work in cooperation with each other to drive an electricalload besides a motor, as long as an input stage of each drive circuit isprovided with a capacitor.

Further, the power supply current monitoring device according to thepresent disclosure can be applied to a load drive apparatus where threeor more systems, each of which has a drive circuit connected in parallelto a power supply. In this case, the present disclosure can be appliedby considering one of the systems as the temporary abnormal system andthe others of the systems as the normal system. In each normal system,the capacitor discharge current Jc having a negative value occurs whenthe power supply relay of the temporary abnormal system is turned ON. Adetection error caused by the capacitor inrush current Ic from eachnormal system can be prevented in the same manner as disclosed in theembodiments.

Such changes and modifications are to be understood as being within thescope of the present disclosure as defined by the appended claims.

What is claimed is:
 1. A power supply current monitoring device for aload drive apparatus, the load drive apparatus including a battery andtwo systems configured to work in cooperation with each other to drive aload, each system including a drive circuit connected in parallel withthe battery, an input capacitor connected between the battery and thedrive circuit, and a power supply relay connected between the drivecircuit and a power blanching point, at which power of the battery isdivided between the systems, to open and close a power supply path fromthe battery to the drive circuit, the power supply current monitoringdevice configured to monitor a magnitude of a power supply currentflowing through the power supply relay toward the drive circuit in eachsystem, the power supply current monitoring device comprising: a failuredeterminator that determines whether a short-circuit failure occursbased on a voltage across a current detector through which the powersupply currents flows, the short-circuit failure defined as a failureoccurring when a high potential side and a low potential side of thedrive circuit is short-circuited, and a power supply relay controllerthat opens and closes the power supply relay, wherein when the failuredeterminator detects the power supply current exceeding a predeterminedfirst threshold once in one of the systems, a repetitive monitoringprocess is performed, the one of the systems being defined as atemporary abnormal system having a possibility that the short-circuitfailure occurs therein, and the other of the systems being defined as anormal system having no possibility that the short-circuit failureoccurs therein, the repetitive monitoring process determines that theshort-circuit failure actually occurs in the temporary abnormal systemwhen a predetermined condition is satisfied by repeating a monitoringcycle a predetermined number of times, the monitoring cycle turns OFFthe power supply relay of the temporary abnormal system and thenmonitors the magnitude of the power supply current when a predeterminedcycle time elapses from a start time by turning ON the power supplyrelay of the temporary abnormal system, the start time being a time atwhich the power supply current exceeds the first threshold, themonitoring cycle accumulates an overcurrent time during which the powersupply current remains above the first threshold after the power supplyrelay is turned ON, the power supply current to be monitored in themonitoring cycle when the power supply relay of the temporary abnormalsystem is turned ON contains a capacitor inrush current and a batteryshort-circuit current, the capacitor inrush current being a currentflowing from the input capacitor of the normal system into the temporaryabnormal system, the battery short-circuit current being a currentflowing from the battery into the temporary abnormal system when theshort-circuit failure continues in the temporary abnormal system, thecondition is satisfied when the monitoring cycle in which theovercurrent time reaches a predetermined second threshold is repeated apredetermined consecutive number of times, the second threshold isgreater than a first overcurrent time and smaller than a sum of thefirst overcurrent time and a second overcurrent time, the capacitorinrush current remains above the first threshold during the firstovercurrent time, and the battery short-circuit current remains abovethe first threshold during the second overcurrent time.
 2. The powersupply current monitoring device according to claim 1, wherein therepetitive monitoring process performs a mask procedure at least duringa predetermined time period between the power supply relay of thetemporary abnormal system is turned ON and then turned OFF, and the maskprocedure stops monitoring the power supply current in the normalsystem.
 3. The power supply current monitoring device according to claim1, wherein the drive circuit, the input capacitance, the power supplyrelay, and the current detector of one system are identical inspecification and electric performance with the drive circuit, the inputcapacitance, the power supply relay, and the current detector of theother system.
 4. The power supply current monitoring device according toclaim 1, wherein the drive circuit is an inverter configured to drive anAC motor or an H-bridge circuit configured to drive a DC motor.
 5. Thepower supply current monitoring device according to claim 4, wherein theAC motor or the DC motor is included in an electric power steeringsystem of a vehicle and outputs steering assist torque that augments adriver's steering effort of a steering wheel of the vehicle.